The embodiments of the present invention relate generally to methods for characterizing a subterranean formation surrounding a wellbore. More specifically, the embodiments relate to methods for characterizing the formation using data obtained from wellbore logging.
Fluids stored in subterranean formations are contained, often at elevated pressures, within pores found within the formation rock. The removal of these fluids from subterranean formations during the production of hydrocarbons, native water, injected fluids, or steam results in a decrease of pore pressure within the formation. This decreased pore pressure leads to a lowering of mechanical support provided to the rock system and can result in closer packing of formation particles or in some cases the movement and/or removal of formation particles by the production processes.
If the formation loses enough mechanical support, portions of the formation yield and break. This is known as formation compaction. When formation compaction occurs, the portion of a wellbore through the compacted formation can be affected. Thus, it is often desirable to monitor the compaction within a producing formation and control the production processes to limit damage to the formation or wellbore. Additional value in monitoring formation compaction may be derived by providing additional wellbore lifetime and providing a prediction of the time for which a wellbore will be mechanically able to support commercial or scientific activities.
One method currently used to monitor formation compaction involves placing marker tags, normally consisting of a radioactive material, onto the casing at known intervals. These tags are typically placed on the casing before it is run into the well or into the formation before or after running the casing into the wellbore. In some applications, marker tags may be installed into an existing casing already in place in the wellbore. The intervals between the marker tags can then be monitored by sensors, such as gamma ray detectors, run into the wellbore on a downhole tool. This process is discussed in “GOM Offshore Subsidence Monitoring Project with a New Formation Compaction Monitoring Tool”; Ame de Kock, Shell Offshore Inc. New Orleans, La.; T. Johnson, Halliburton Energy Services, New Orleans, La.; T. Hagiwara, H. Zea, F. Santa Halliburton Energy Services, Houston, Tex., which is hereby incorporated by reference herein for all purposes. Although providing a direct measurement of casing deformation, which is related to and caused by formation compaction, many wells do not have the marker tags required to perform the measurement. Additionally, because the use of radioactive materials is heavily regulated, non-radioactive solutions are desirable.
It is known that formation compaction can cause damage to the casing contained within the wellbore. As formation compaction occurs, the casing is compressed. This compression can lead to changes in the casing's diameter, thickness, and roundness as well as cause large diameter bends in the casing. In extreme cases, the casing fails, thus disrupting production from the well. Thus, it is desirable to monitor casing mechanical deformation and formation compaction in order to provide early detection of formation compaction problems, allowing the reservoir management procedures to be changed accordingly. Well lifetime mechanical conditions and dynamic predictions allow optimized strategic planning for the existing well and also the best planning for replacement wells as needed.
The collection of downhole information, also referred to as logging, is realized in different ways. Logging is used to measure many different properties of the casing, wellbore, and surrounding formation. Tools to measure wellbore properties may employ techniques involving electromagnetic signals, ultrasonic signals, refracted or flexural sonic signals, nuclear radiation sources, and mechanical measurements. For example, ultrasonic imaging acquisition has been used to help determine the deformation of the well casing by transmitting ultrasonic signals into the well and analyzing their reflections. Through this ultrasonic measurement information about the wellbore, casing, cement, and formation can be determined. Techniques for using ultrasonic data to compute borehole geometry are disclosed in U.S. Pat. No. 5,638,337 and U.S. Pat. No. 5,737,277, both of which are incorporated by reference herein for all purposes.
It is also known in the art to mechanically measure the diameter, also known as the caliper, of a borehole to correct formation measurements that are sensitive to size or standoff. These corrections are necessary for accurate formation evaluation. One technique for measuring the caliper incorporates a mechanical apparatus with extending contact arms that are forced against the wall of the borehole.
Thus, there remains a need in the art for methods of characterizing a subterranean formation using data acquired during well logging activities. Therefore, the embodiments of the present invention are directed to methods, of correlating well logging data into useful data for evaluating and characterizing the formation, that seek to overcome the limitations of the prior art.